Electrostatic fluidized bed coating is, of course, a conventional and widely used technique for depositing particulate materials upon a great diversity of workpieces. Typical of the apparatus used for that purpose are the devices disclosed in Knudsen U.S. Pat. No. 3,916,826 and in Karr U.S. Pat. No. 4,030,446, While electrostatic coating with such devices is highly efficient, effective and safe, fluidized bed coating in general is not without its limitations and difficulties.
A particularly difficult problem relates to the attainment of uniform deposits upon all surfaces of the articles being coated. Such problems are due in part to the effects of the workpiece configuration upon the nature of the coating produced. Thus, a non-uniform workpiece will tend to develop an irregular deposit, particularly when, for example, the article has sharp edges or elements of relatively small dimension. But even when the workpiece is of entirely uniform configuration and is free of edges (e.g., when it is a length of round wire), the proximity of other workpieces will usually have an effect. Thus, when a plurality of wires are coated simultaneously, stopping or removing one of them will usually significantly change the characteristics of the deposit produced upon the others. This is highly undesirable in an automated operation, such as is, as a practical matter, often necessary in the commercial production of insulated wire. But even when only one wire or conductor is involved, still a problem exists in producing the high degree of uniformity required for many applications, and this is particularly true when the conductor is of non-circular (e.g., rectangular) cross section.
In those instances in which coatings are produced upon articles moving horizontally (or substantially so) above or through a fluidized bed, the difficulty of producing uniformity is compounded by the fact that rarefaction occurs upwardly within the cloud. As a result, the upper surfaces of the articles are exposed to less powder than are the lower portions, and therefore the deposits developed thereon tend to be thinner. The generally upward movement of the particles of the cloud, under the influence of the gas passing upwardly through the porous support plate of the fluidized bed, also favors the development of heavier coatings on lower surfaces.
Attempts have been made to compensate for the foregoing characteristics of electrostatic fluidized bed coating, such as through the use of appropriate masking devices or baffles to block those surfaces which would otherwise tend to receive disproportionately large amounts of the powder; such apparatus is, for example, disclosed and claimed in Goodridge U.S. Pat. No. 3,828,729. In Westervelt et al U.S. Pat. No. 4,011,832, tubular build control means is disclosed for the purpose of controlling the uniformity of the thickness of deposits upon a workpiece. While those approaches have considerable merit, the equipment involved is of limited flexibility, and therefore not optimally suited to certain applications.
Many of the limitations inherent in prior art apparatus and methods have been alleviated or avoided by the inventions set forth in the following United States applications for Letters Patent, each of which is common assignment herewith: Ser. No. 14,656, filed in the name of Donald J. Gillette on Jan. 23, 1980 and now issued as U.S. Pat. No. 4,240,986, which application is entitled CONTROL GRID IN ELECTROSTATIC FLUIDIZED BUILD COATER; Ser. No. 218,521, filed on Dec. 23, 1980, also in the name of Donald J. Gillette, and now issued as U.S. Pat. No. 4,330,567, which application is entitled METHOD AND APPARATUS FOR ELECTROSTATIC COATING WITH CONTROLLED PARTICLE CLOUD; and Ser. No. 218,522, filed on Dec. 23, 1980 in the name of Walter G. Knudsen and now issued as U.S. Pat. No. 4,332,835, which application is entitled PLENUM MOUNTED GRID FOR ELECTROSTATIC FLUIDIZED BED. However, further improvement, especially in regard to uniformity of the deposit on workpieces of non-circular cross section, and in regard to the facility with which variations can be produced to accommodate different workpieces and to optimize the coating, are of course desirable.
Thus, it is a principal object of the present invention to provide novel build control means, apparatus, systems and methods, for electrostatic powder coating, by which workpieces, and especially wires and other electrical conductors of continuous length, can be coated with a high degree of uniformity, or controlled variation, in the thickness of the deposit.
It is a more specific object of the invention to provide novel build control means, and apparatus, systems and methods utilizing the same, which variations in the nature of the deposit produced can readily and conveniently be made, so as to achieve optimal results tailored to the characteristics of the workpiece being coated.
Another object is to provide the foregoing by means that are relatively simple and inexpensive to construct, and convenient and facile to utilize.